I'm not talking about case hardening.

Pretty much any piece of metal that's been worked will form what's known as an 'Alpha Case'

The alpha case is essentially a very thin skin-like layer of the material where the grain structure of the material is more compacted and/or aligned than the remaining parent material.

Bluing and honing, and definitely shooting will impart an alpha case on good structural steels like those used in gun barrels.

Case hardening is a different thing entirely. Color case hardening even more so.

--Tinker